Tagged Questions

The study of the large-scale structure, history, and future of the universe. Cosmology is about asking and answering questions about the "big picture" - the extent, origin, and fate of everything we know.

Bottom up is the main stream science view. Most models of the universe are bottom-up, that is, you start from the well-defined initial conditions of the Big Bang and work forward.
The other view is ...

I am trying to obtain an expression for the power spectrum $\Delta (k)$ of cosmological perturbations, see for example eqn (148) from the following TASI lectures.
Right now, I would like to use the ...

I'm trying to understand the connection between the $\Lambda$ from cosmology and the $\Lambda$ from QFT.
Cosmology: The cosmological constant enters the Einstein equations. In the special case of the ...

Recently I watched a minute-physics video that suggested that a better name for the beginning of time would be "Everywhere stretch" because there wasn't a space-time singularity that formed where the ...

The largest known galaxy is just 100x more massive than Milky Way.
I wonder if there is some physical mechanism (some equilibrium) which limits the size of galaxies or if it is just because of limited ...

Galaxies are moving away from us proportional to the distance between us and them , but nothing can travel faster than light, so even the farmost galaxies should be travelling away from us along with ...

In the gravitational redshift, the frequency of photons radiated from some source is reduced. As the energy of a photon is given by $\hbar\omega$, if the frequency is reduced where is the lost energy?
...

Are there any hypotheses on the expansion of the universe, which have a cosmological constant, or some other parameter, that results in a universe with an ever increasing rate of acceleration for the ...

I'm an undergraduate student currently following a particle astrophysics lecture. I was reading some stuff about the neutrinos decoupling before the $e^+ e^-$ annihilations. It's possible to quickly ...

I'm trying to follow the calculations in http://arxiv.org/abs/hep-th/0201158v2 The aim is to rederive the expressions (2.16), (2.17) for the power spectrum in de Sitter spacetime. In order to do so, ...

The way I understand the holographic principle is that everything in a 3D space can be thought of as living on the 2D boundary of that space. If that is the case, why does everything in the universe ...

As far as I know is the discovery that galaxies that are farther away are moving faster from us than galaxies the are closer by. This led to the theory of Dark Energy. The acceleration was speeding ...

The light cone of our galaxy is a lot different to the light-cone of a galaxy 5 bly away. Our Hubble volumes are much different. Everyone is born in/at a different space/time.
Can I see stars in my ...

Space is expanding and as we know space and time are intrinsically linked to be now known as spacetime.
What is happening to time during expansion? Is there more time, longer time or is the time part ...

Currently (2015), what is the total mass of the universe? How can we say with mathematical certainty that 69% is dark energy; 25% is dark matter; 5% is atomic matter; and 1% is neutrino, photon, and ...

In common interpretations of quantum mechanics, it can only be said that objects exist once I observe them - it is not legitimate to ask where an object was before I observed it.
Does this point of ...

Let us consider the total number of relativistic degrees of freedom $g(T)$ for particle species in our universe:
$$g(T)=\left(\sum_Bg_B\right)+\frac{7}{8}\left(\sum_Fg_F\right)$$
Where the sums are ...

If one models the electron as a hollow spherical conductor with charge $e$ and radius $a$ then its electrostatic energy is given by:
$$E_{em}=\frac{1}{2}\frac{e^2}{4\pi\epsilon_0a}$$
However if one ...

When speaking with researchers and professors at my physics department, I sometime catch this undertone in the conversation when speaking about GR: almost as if some are not entirely convinced about ...

As Im sure many fo you there are many attempts at singularity resolution in quantum gravity approaches such in LQC and string theory. In the last few week there was a lot of press for a new approach ...

The Cosmic Microwave Background anisotropy is currently aligning Earth and the Solar System with the largest and earliest structure of the universe. Has this been explained yet? Or are the theories ...

The reason I am asking this question is because if all points in space observe recession of galaxies the same as we do from Earth, the universe would have to be infinite (or a closed sphere in 4D or ...

Only partly kidding...
Let's suppose that a thousand, or a million, years ago there was an explosion of some gigantic star (or other celestial object) that was strong enough to destroy Earth and Life ...

There is a possibility for our universe to be the surface volume of a (higher-dimensional) hyperspace. So if this possibility is true,then is there also a possibility that the other galaxies we see ...

If I point a light torch to a cloudless, crystal clear and an absolutely pristine sky, will the light beam exit the planet and travel forever in the vacuum of space? (assuming it doesn't get perturbed ...

The question I have is: Why is space (almost perfectly) flat in our neighbourhood? (I am disregarding the deviations due to the sun and the planets.)
Is it correct to say that space is (almost) flat ...